1. Field of the Invention
This invention relates to power supply controllers, and particularly to digital power supply controllers.
2. Description of the Related Art
Power supplies typically comprise a power stage and a controller, with the controller arranged to operate the power stage as needed to produce a desired output voltage. A power stage can be, for example, a pass transistor (for a linear power supply) or a switching circuit which drives an output inductor and capacitor (for a switching power supply).
A typical switching power supply which includes an analog controller is shown in FIG. 1a. In this example, power stage 10 includes a switching circuit 12 which drives an output inductor L1 and capacitor C1, to produce an output voltage Vout. The controller includes an analog error amplifier 14 which receives Vout at one input and an analog reference voltage Vref at a second input; the value of Vref is typically programmed by a potentiometer 16. Note that, typically, only a fraction of Vout is provided to amplifier 14, via a voltage divider (not shown for simplicity). The resulting error voltage Verr is filtered (18) and used to drive a pulse-width modulation (PWM) circuit 20, the output of which controls the operation of power stage 10. Power supplies of this type are widely used; however, they are limited in that, because the controller is analog, reference voltage Vref and the characteristics of filter 18 cannot be easily changed using computer software.
One possible digital controller is shown in FIG. 1b. Here, the analog error amplifier is replaced with an analog-to-digital converter (ADC) 30. The reference voltage is subtracted from the output of the ADC to produce an error voltage, but here, the reference voltage Vref is represented digitally, and the subtraction of the reference voltage and the filtering (32) are both done in the digital domain. However, though both the reference voltage and filter characteristics can be changed using software, the ADC typically requires both high-bandwidth and high-resolution characteristics, making it an expensive building block for this solution.
Another approach is shown in FIG. 1c. Here, an analog error amplifier 40 is imposed between Vout and an ADC 42; by so doing, the resolution of the ADC can be much lower than that required in FIG. 1b. However, to retain the option of programming reference voltage Vref, a high-resolution digital-to-analog converter (DAC) is required.